This invention relates to the art of investment casting, and more particularly to new and novel composition and combinations of shell building materials to provide an improved shell mold.
The standard methods of forming a ceramic shell mold over a heat destructable pattern are well known in the art. Basically these steps include the following:
(1) A set-up of heat destructable patterns, such as wax patterns, is prepared on a suitable runner system.
(2) A primer slurry is prepared consisting of a finely divided (about 200 to 325 mesh) refractory powder or "flour" (fused silica, zirconium silicate, or calcined alumino-silicate) and a binder such as 15% to 30% silica sol, plus a small addition of wetting agent and antifoam agent. Ethyl silicate binders are also used. These are blended in suitable proportions to form a slurry which is kept in suspension by mechanical agitation.
(3) The set-up is dipped into the slurry to coat all surfaces. The set-up is then withdrawn and the excess slurry allowed to drain off. The set-up is manipulated either by hand or mechanically so as to promote as nearly uniform a coverage of the set-up and patterns by the refractory slurry as possible.
(4) After draining to a uniform coverage and while the refractory slurry is still wet, the entire assembly is treated, either by sprinkling on or by use of a fluid bed, with a stucco material. That stucco is a refractory grain such as calcined alumino-silicate (Molochite or Mulgrain), fused silica, zirconium silicate or aluminum oxide. The particle size of these stucco grains is usually not coarser than a U.S. 50 mesh and on a U.S. 120 screen. The major portion will be through a U.S. 50 mesh and on a U.S. 120 screen.
(5) The first slurry coat is now allowed to dry under strictly controlled temperature and humidity conditions.
(6) After drying and prior to the second dip coat the entire set-up is immersed in water or in a pre-wet solution of 15% silica sol plus a small addition of wetting agent. The purpose of the pre-wet dip is to ensure a uniform coverage of the slurry coated patterns without the formation of entrapped air bubbles.
(7) After quickly immersing the set-up in the pre-wet solution (in and out immediately) the excess pre-wet solution is allowed to drain off for about 30 to 60 seconds. Steps 3, 4 and 5 are then repeated. The slurry used for the second dip is the same as for the first coat.
(8) The remaining slurry coats are applied using a socalled "back-up slurry". The composition may be similar to the slurry used for the primer coats, however, the particle size of the refractory flour in the slurry is usually a little larger. The stucco material applied to the back-up coats is usually considerably coarser that that used for the primer coats. A typical sieve analysis for a suitable back-up stucco is as follows: about 20% on a U.S. 20 screen, about 40% on a U.S. 30 screen, about 30% on a U.S. 40 screen, about 10% on a U.S. 50 screen, and not to exceed about 2% through a U.S. 50 screen. Actual particle size distribution may vary considerably and suitable stucco materials are generally of a size through a U.S. 18 screen and on a U.S. 50 screen.
(9) The steps of dipping, draining, stuccoing, and drying are repeated until the desired shell thickness is obtained. Usually about five more dips are applied for a total of seven coats, although more or fewer coats may be used depending upon the size and configuration of the parts to be cast and the total weight of the metal to be poured into the finished shell. The pre-wet step used between the first and second slurry coat may also be used between the second and third slurry coat. Although additional pre-wet steps may be used in special circumstances, it is not ordinarily required for the production of satisfactory shell molds.
After completing the series of dipping, draining, stuccoing and drying steps the finished shell mold is dried thoroughly, usually overnight. This is followed by the usual steps of dewaxing, firing, casting, cooling, knock-out, salt bath, cut-off, sand blasting and other necessary after cast operations all well known in the art.
The materials, processes, and techniques generally used as described above are not completely satisfactory in many respects and there is considerable room for improvement in several areas.
For example, the ceramic shell mold becomes very hard after firing and casting. Thus the removal of the ceramic mold material to recover the metal castings becomes very difficult and expensive. A pneumatic hammer is usually employed to remove as much refractory shell material as possible from the cast tree. In some instances the mechanical vibration is so severe that the metal castings are cracked or otherwise damaged.
After the knock-out operation there usually remains a considerable amount of refractory shell material that must be removed. The is particularly true for set-ups of relatively small parts that are spaced closely together resulting in a dense packing of solid refractory material.
In addition, parts having holes, slots, and internal configurations usually trap shell material which many times cannot be removed by mechanical vibration. Usually a molten salt bath of caustic soda (sodium hydroxide) is used to dissolve the remaining shell material. This, too, is a time consuming and expensive operation. The caustic soda soon becomes neutralized or spent and will no longer remove the shell material from the casting. The salt bath must be cleaned and rejuvenated by removing the sludge and discarding a portion of the spent salt, so that fresh sodium hydroxide can be added. This is a hot, dirty, and hazardous operation. Moreover, the safe disposal of the spent caustic soda is now an increasingly serious environmental problem.
Another area needing improvement has to do with the permeability of the shell mold. A high permeability is needed to permit the hot gases in the mold to escape as the molten metal is cast into the mold.
Still another area for improvement is in the combination of a shell mold with extremely high permeability together with very good strength characteristics.
One attempt to solve some of these difficulties has been to substitute a special refractory slurry for one or more of the intermediate layers. Usually this has been accomplished at the third or third and fourth dip coats. It has long been known to prepare slurries comprising a refractory powder and an organic binder such as polyvinyl alcohol. See U.S. Pat. No. 3,165,799 issued Jan. 19, 1965, Column 5, lines 32 and 33, and lines 44 to 55. See also U.S. Pat. No. 3,903,950 issued Sept. 9, 1975, and U.S. Pat. No. 2,912,729, issued Nov. 17, 1959.
An intermediate layer or layers such as this bound only with an organic binder will revert to a loose, free flowing powder after heating to a high enough temperature to burn off and remove the organic binder. This approach does indeed improve the ease with which the bulk of the ceramic shell material is removed by mechanical vibration after casting. However, it is not entirely satisfactory. One difficulty is that the one or two primer coats that are applied prior to the intermediate coats containing the organic binder polyvinyl alcohol are not attached to the back-up coats. Thus, there is a loosely filled space or gap in the mold. Many times the force or weight of the molten metal will cause the first two coats (precoat layers) to buckle into the gap or space left by the intermediate layer. This results in a defective casting. Furthermore, the overall strength of the shell mold is reduced because the intermediate layer produces a laminating effect with no bond between the primer layers and the back-up players.
Another difficulty arises if the ceramic mold should crack during any of the processing steps. The free flowing refractory powder tends to flow out of the interior of the built up shell construction and may be deposited within the mold cavity or cavities, causing ceramic inclusions in the metal casting.
Another approach to solve some of these difficulties has been to attempt to use expanded or foamed plastic (poly-styrene) beads or particles as a stuccoing material. See U.S. Pat. No. 3,362,463 issued Jan. 9, 1968. This process is not used in production, but only occasionally for certain special applications. This is presumably because, as my experiments have shown, it is difficult to apply the particulate material to produce a satisfactory coat. When the application is attempted by sieving, the foam particles do not go through the screen easily, and those which do generally will come lightly to rest on the surface of the slurry rather than become embedded in it. Moreover, they are not deposited on the internal cored surfaces. If the set-up is plunged into a bed of foam particle stucco, other difficulties ensue. The particles adhere to each other, blocking openings to cored passages and blocking particle access to some portions of external surfaces. The use of this material is therefore limited to simple shapes having external surfaces only and with parts spaced sufficiently far apart so that contact of the foamed polystyrene with the wet slurry surface is possible.
Yet another problem in the investment casting industry has been the rapid growth of that industry and a consequent lack of capacity in manufacturing facilities to produce stucco economically and in sufficient quantities.
In the past, stucco materials such as Molochite, Calamo, Flintgrain, Mulgrain, and fused silica have been widely used for the production of ceramic shell molds. Molochite, Calamo, Flintgrain and Mulgrain* are all trade names for refractory materials and stuccoes that can be described chemically as calcined alumino-silicates. They have found widespread use because they are highly refractory and they have a fairly low and uniform coefficient of thermal expansion. A typical example of the linear thermal expansion of a built-up shell mold comprising seven slury coats and seven stucco layers will show an approximately straight line curve from room temperature to 2000.degree. F. and an increase in length of about 0.4%. This low and uniform thermal expansion is very desirable in shell molding because it minimizes cracking of the shell mold during the firing cycle due to possible differential thermal expansion. FNT * Molochite is prepared from a source of very pure clay mined in England and is available in this country from Casting Supply House in New York. Calamo and Flintgrain are available from Harbison Walker Refractories, Inc. and Mulgrain Stuccoes are products of C.E. Minerals and are available from Pre-Vest, Inc. Cleveland, Ohio.
Quartz occurs naturally in large quantities in the United States. Also, there are large deposits of very pure, high grade quartz. And it is very economical.
However, it has not been used to any great extent in the investment casting industry for shell molding, because of its high thermal expansion. An abrupt thermal expansion occurs as quartz undergoes a transformation from low quartz to high quartz at the inversion temperature of 573.degree. C. (1063.degree. F.). This abrupt increase in volume approximates about 0.9%. The total volumetric expansion which occurs between 600.degree. F. and 1100.degree. F. is about 3.2%. The coefficient of volume expansion for solids is approximately three times the linear coefficient. Therefore the total linear expansion of quartz which occurs between 600.degree. F. and 1100.degree. F. can be figured at about 1.06%. This high coefficient of thermal expansion, and particularly the abrupt increase in volume as the quartz undergoes transformation from low quartz to high quartz, has precluded the use of this material as a suitable refractory for making shell molds heretofore.
In accordance with the above, it is an object of my invention to provide an artificial stucco the use of which results in a shell mold which may be more readily removed than heretofore.
Another object of my invention is to provide a stucco which if desired may be used to produce shell molds with both high permeability and good strength.
A further object of my invention is to provide a stucco which is as easy to use as stuccoes previously known but has the desirable qualities above listed.
Yet another object of my invention is to provide a stucco with favorable thermal expansion properties, but which can be made of economical raw materials which themselves do not possess such favorable properties.